1. Field of the Invention
The present invention relates to a moving coil type loudspeaker, and more particularly to a loudspeaker in which two magnets are disposed with the same poles being faced each other and a voice coil is disposed in a repulsion magnetic field generated by the magnets.
2. Related Background Art
Various types of loudspeakers have been proposed heretofore, in which two magnets are disposed with the same poles being faced each other and a voice coil is disposed in a repulsion magnetic field generated by the magnets (for example, refer to Japanese Patent Laid-open Publications Nos. 59-148500 and 1-98400). One example of such loudspeakers is shown in FIG. 10. Two magnets M1 and M2 magnetized in the thickness direction are disposed with the same poles being faced each other, and a center plate P is sandwiched between the magnets. A tape or the like made of magnetic material F such as amorphous material is wound on the outer circumference of a voice coil 1 wound with a coil made of conductive material C such as copper. This voice coil 1 is disposed spaced apart from the magnets M1 and M2 by a predetermined distance.
Not only a coil but also an amorphous metal tape is required for the voice coil 1 shown in FIG. 10. Therefore, the number of components increases. Furthermore, an amorphous metal tape is difficult to obtain and is expensive more than soft magnetic material such as iron and Permalloy. An amorphous metal tape has generally a high elastic modulus so that it is difficult to curl it and make it match the outer circumferential shape of the voice coil 1.
In order to solve the above problems, the prevent inventors have proposed a loudspeaker such as shown in FIG. 8. In this loudspeaker, two magnets M1 and M2 magnetized in the thickness direction are disposed with the same poles being faced each other, and a center plate P of a disk type made of iron is sandwiched between the magnets. An outer ring L made of soft magnetic material F such as iron is disposed outside of the center plate P, with a predetermined magnetic gap G being formed between the outer circumference of the center plate P and the inner circumference of the outer ring L. A voice coil 1 is disposed in the magnetic gap G.
The present inventors have also proposed a loudspeaker such as shown in FIG. 9 in which the outer ring L is not used but a voice coil 1 is wound with a coil containing magnetic material. For example, the coil may have a core of soft magnetic material F such as iron and a surface layer of conductive material C such as copper and aluminum which is adhered to the surface of the core by means of plating, pressure attaching, or vapor deposition, or it may have a core of conductive material C such as copper and aluminum and a surface layer of soft magnetic material F such as iron and Permalloy which is adhered to the surface of the core by means of plating or vapor deposition.
The magnetic flux distribution of a magnetic circuit in the repulsion magnetic field will first be described. Repulsive magnetic fluxes flow from the magnets M1 and M2 whose same poles (N poles) being faced each other toward the center plate P, radiate out of the outer circumference of the center plate P, and flow immediately to the opposite poles (S poles).
Also in the case of the loudspeaker shown in FIG. 8 having the outer ring L and the loudspeaker shown in FIG. 9 without the outer ring L and with the voice coil 1 containing magnetic material F, part of magnetic fluxes radiated from the outer circumference of the center plate P immediately flows to the opposite poles (S poles), and most of the magnetic fluxes pass through the outer ring L or the magnetic material F and flow to the opposite poles (S poles).
Therefore, the magnetic flux distribution of the magnetic circuit becomes as shown in FIG. 11. Specifically, the quantity of fluxes is large near at the center plate P, and it reduces at the positions upper and lower than the center of the center plate P. The quantity of fluxes becomes zero near at the position about 1/3 to 1/2 the width of each magnet. At the positions upper and lower than the zero points, the flow of fluxes becomes opposite to that near the center plate P. The quantity of negative fluxes increases and becomes maximum at the top and bottom of the magnets M1 and M2. At the positions upper and lower than the maximum points, the quantity of fluxes converge to zero. In other words, although fluxes sufficient for driving the voice coil 1 are generated near at the center of the center plate P, at the positions upper and lower than the center plate P, negative fluxes are generated which suppress the normal operation of the voice coil 1.
A loudspeaker using a magnetic circuit with a repulsion magnetic field becomes sufficient for practical use if the winding width of a voice coil is set within a predetermined range. However, obviously, it is more preferable if a magnetic circuit has no negative flux. A conventional repulsion magnetic circuit is very difficult to eliminate negative magnetic fluxes.